1. Field of the Invention
The invention relates to charge injection devices (CID) for sensing IR image intensity information obtained from a two dimensional array of dual-gate sensing sites on an InSb or HgCdTe substrate, and more particularly to a novel push-pull readout circuit which eliminates the pedestal due to capacitive coupling between gates on the same pixel of a dual gate CID.
2. Prior Art
Dual-gate charge-injection device focal plane mosaics are solid-state self-scanned sensors that employ surface charge transfer/injection to achieve full X-Y address capability for area arrays. The pixels are conventionally scanned one row at a time, with separate parallel processing of the columns which is later multiplexed into a serial format. The charge transfer takes place between the row and column sites (and vice versa) at a dual-gate site. Injection represents the injection of charge into the substrate from the dual-gate site accompanied by a flow of charge at the input of the column preamplifier of the readout circuit.
The now well-known Sequential Row Inject (SRI) readout is achieved by placing the inject pulse on the rows, one at a time, and sensing the injected charge on the column. As usual, the desired signal voltage is the change in column voltage as a result of the injected charge, and is obtained by sampling the column voltage before and after the injection pulse by correlated double sampling. A bias charge is always maintained in the potential wells of both row and column.
The success of SRI lies in the fact that charge transfer/injection occurs only once in the selected cell (row) of each column in the sequence of scanning all the rows. The other cells are left undisturbed, thus eliminating the possibility of charge contributions from them and providing greater tolerance to charge-transfer inefficiency due to the classical InSb/oxide interface problem.
The conventional approach to implementing the SRI readout mode in a pixel readout cycle is illustrated in FIG. 6C. The pixel readout cycle starts with a reset of brief duration (tr). The first sample of the correlated double sampling (CDS) process is taken after the waveform has settled from the reset disturbance (t1). The injection (ti) is applied right after the completion of the first sample with only a minimal delay which is required to insure that the leading edge of the injection pulse does not overwhelm the first sample. After injection, an additional delay (t2) is required to allow the trailing edge of the injection pulse to settle (the pause time determines the pre-CDS bandwidth) before the second sample is taken, which concludes the pixel readout cycle. The total pixel readout time tp is then the sum of tr, t1, ti, and t2. For a sequential readout, tp=Ti/m, Ti and m being the integration time (or line time) and number of pixels (rows) in the column, respectively.
With the growth in the size of the arrays from 32.times.32 to 128.times.128 to 256.times.256 the quantity m grows from 32 to 128 to 256. On the other hand, the integration time Ti is specified by the system requirement, but is limited to roughly 1.0 millisecond due to array dark current and storage capacity constraint. As a consequence, the available pixel readout time tp is no longer adequate for the normal readout process. Specifically, the injection time ti needs to be at least 1.0 microsecond to minimize the lag caused by incomplete charge injection, whereas low preamp noise requires long settling time (t1 and t2) for narrow pre-CDS bandwidths. Accordingly, the faster the readings, the higher the bandwith requirement of the preamplifier and the greater the sensitivity to input noise.
In the actual implementation of the conventional SRI readout, there are other complications. High performance sensor systems normally use CMOS parallel video processor (PVP) chips on the focal plane. The coupling of the injection pulse from row to column (preamp input) causes a pedestal which is usually large enough to saturate the amplifier chain. Consequently, waveform settling does not start during the "dead time" until the amplifiers recover from saturation.
The problem forces the preamplifier to be designed to handle a greater dynamic range. The other complication associated with the sample-after-inject readout is the CID noise due to tunneling breakdown which reduces the ultimate sensitivity of the array to weak IR radiation.